A DC-to-DC converter converts an unstable input voltage of direct current to a stable direct current output voltage to output it to a load. DC-to-DC converters are used as power supplies of various electronic devices that operate on direct current voltage. DC-to-DC converters are separated, based on the operation principle, into an insulation type that increases and reduces the input voltage with a transformer and a non-insulation type that interrupts a current flowing through an inductor with a switching transistor and converts a direct current input voltage to a direct current output voltage of a different voltage level and polarity. The electronic devices in which the input voltage and the output voltage are not greatly different employ non-insulation type DC-to-DC converters that can be configured of relatively simple circuit elements.
A related step-down DC-to-DC converter 100 described in Japanese Patent No. 5811237 is described below with reference to FIG. 3. The DC-to-DC converter 100 includes a direct voltage conversion unit 10 that steps down an input voltage Vi of direct current and outputs it between a high voltage-side output terminal 32a and a low voltage-side output terminal 33a, which are connected to a load RL, and an abnormality determination circuit 4 and a protection circuit 7, which are described below. A direct current input power supply 30 generates the input current Vi of direct current between a high voltage-side power supply terminal 30a and a low voltage-side power supply terminal 30b. As illustrated, a diode D1 of which forward direction is from a low voltage side to a high voltage side and a switching transistor Tr1, the diode D1 and the switching transistor Tr1 being in the direct current voltage conversion unit 10, and a switching transistor Tr2 are connected in series between the high voltage-side power supply terminal 30a and the low voltage-side power supply terminal 30b to form a closed circuit.
A connection point A between the diode D1 and the switching transistor Tr1 is connected via an inductor L1 to a high voltage-side connection line 32, on one side of which is the high voltage-side output terminal 32a. Moreover, the other side of the connection point A of the diode D1 is connected to a low voltage-side connection line 33 wired between the low voltage-side power supply terminal 30b and the low voltage-side output terminal 33a. A capacitor C1 is connected between the high voltage-side connection line 32 and the low voltage-side connection line 33 to supply stable direct current power of an output current Io and an output voltage Vo to the load RL connected between the high voltage-side output terminal 32a and the low voltage-side output terminal 33a. 
The switching transistor Tr1 comprises an FET (field-effect transistor). A drive signal output from a constant voltage control circuit 40 to a gate of the switching transistor Tr1 controls the opening and closing of the switching transistor Tr1. Assume that the switching transistor Tr2 serving as the protection circuit 7 is controlled to be normally closed (ON control). While the switching transistor Tr1 is being controlled to be closed (ON control) and is operating in a saturation state, a current flows from the direct current input power supply 30 to the inductor L1 to charge the capacitor C1. However, the charge voltage of the capacitor C1 becoming the output voltage Vo is a voltage lower than the input voltage Vi due to the self-inductance of the inductor L1. Moreover, while the switching transistor Tr1 is being controlled to be open (OFF control) and is operating in an interruption state, electrical energy stored in the inductor L1 becomes a charge current that is fed back through the diode D1 to charge the capacitor C1 and maintain the charge voltage of the capacitor C1 becoming the output voltage Vo.
In terms of the output voltage Vo, its voltage level can be controlled by the closing control time of the switching transistor Tr1 during a unit time. Accordingly, the constant voltage control circuit 40 provides the negative feedback of the duty cycle of the drive signal to perform the control of closing the switching transistor Tr1 from the output voltage Vo, and brings the output voltage Vo to the operating voltage of the load RL through constant voltage control. Hence, the constant voltage control circuit 40 includes a pair of resistive dividers R1 and R2 connected between the high voltage-side connection line 32 and the low voltage-side connection line 33, causes an error amplifier 41 to compare the voltage of a connection point between the resistive dividers R1 and R2 and a reference power supply voltage Vref that is adjusted to a predetermined potential based on the operating voltage of the load RL, and outputs the result to a pulse-width modulation circuit PWM. The pulse-width modulation circuit PWM modulates the pulse width of an oscillation signal with a constant period output from an oscillator OSC with a comparison signal of the error amplifier 41. The pulse-width modulation circuit PWM outputs the signal to a drive circuit 42. The drive circuit 42 outputs, to the gate of the switching transistor Tr1, the drive signal of which duty cycle has been adjusted in accordance with the comparison signal of the error amplifier 41. Consequently, for example, when the output voltage Vo is higher than the operating voltage of the load RL, the drive circuit 42 outputs the drive signal of which duty cycle has been reduced to the gate of the switching transistor Tr1. Accordingly, the ON control time within the unit time is reduced. Therefore, the output voltage Vo reduces. Conversely, when the output voltage Vo is lower than the operating voltage of the load RL, the drive signal of which duty cycle has been increased is output to the gate of the switching transistor Tr1 to extend the ON control time within the unit time. Therefore, the output voltage Vo increases. Accordingly, the output voltage Vo is brought to a predetermined operating voltage that is different according to the load RL through constant voltage control.
On the other hand, when the pulse-width modulation circuit PWM or the like of the constant voltage control circuit 40 fails due to some cause such as a lightening strike, and the drive circuit 42 outputs the drive signal at a constant potential that puts the switching transistor Tr1 in an active state to the gate (base) of the switching transistor Tr1, the DC-to-DC converter 100 puts the switching transistor Tr1 in a normally closed state (ON state) to operate as a series regulator (dropper circuit) that consumes the input power by the on-resistance of the switching transistor Tr1 and outputs the output voltage lower than the input voltage.
However, unlike a power MOS and a power transistor, which take measures against heat dissipation, a DC-to-DC converter that reduces the switching loss at the switching transistor Tr1 as much as possible and converts the input voltage to a direct current output voltage highly efficiently uses the switching transistor Tr1 that cannot dissipate thermal energy generated by the on-resistance, which becomes a cause of the occurrence of a serious accident where heat is generated to cause a fire. In addition, even if the switching transistor Tr1 operates in the active state, the output voltage and the output current do not fluctuate largely from the set values. Accordingly, the risk of a fire is increased while the abnormality of the switching transistor Tr1 operating in the active state cannot be found from the outside.
Hence, in the DC-to-DC converter 100 described in Japanese Patent No. 5811237, the focus is concentrated on the point that when the switching transistor Tr1 operates abnormally in the active state, the switching transistor Tr1 does not perform the opening and closing operation in a predetermined cycle and a voltage Vd of the connection point A on the side, which is connected to the inductor L1, of the switching transistor Tr1 does not change. When the voltage Vd of the connection point A does not change during a detection period longer than the predetermined cycle, the abnormality determination circuit 4 connected to the connection point A determines that the switching transistor Tr1 is operating in the active state.
Moreover, the switching transistor Tr2 serving as the protection circuit 7 is connected between the high voltage-side power supply terminal 30a and the switching transistor Tr1. The abnormality determination circuit 4 is connected to a gate of the switching transistor Tr2. The switching transistor Tr2 is controlled to be normally closed (ON control) by the drive signal output from the abnormality determination circuit 4. When the abnormality determination circuit 4 has determined that the switching transistor Tr1 is operating in the active state, the switching transistor Tr2 is controlled to be open (OFF control). The supply of current to the switching transistor Tr1 from the direct current input power supply 30 is stopped. Consequently, it is possible to prevent abnormal heat generation in the switching transistor Tr1.
According to the above-mentioned related DC-to-DC converter 100, when the switching transistor Tr1 operates in the active state, the supply of current to the switching transistor Tr1 is stopped, and fail-safe that stops the operation of the direct current voltage conversion unit 10 works. Accordingly, a fire accident due to heat generation in the switching transistor Tr1 can be prevented from occurring. However, fault tolerance is not considered. Therefore, when the abnormal operation of the switching transistor Tr1 is detected, the supply of direct current power to the load RL is also stopped. If the DC-to-DC converter 100 is used in systems for, for example, the flight of an airplane, the travel of an automobile, and the lift of an elevator, direct current power is not supplied to these systems to become a cause of provoking a severer accident with the risk of death.
Hence, when the function of fault tolerance is added to the DC-to-DC converter 100, a redundant power supply circuit having the same configuration as that of the above-mentioned direct current voltage conversion unit 10 is normally placed in parallel between the high voltage-side output terminal 32a connected to the direct current input power supply 30 and the load RL, and the low voltage-side power supply terminal 30b. When an abnormal operation of the switching transistor Tr1 of the direct current voltage conversion unit 10, which is caused to operate normally, is detected, the redundant power supply circuit is activated to operate.
However, if the redundant power supply circuit having the same configuration is placed in parallel in this manner, the entire circuit configuration of the DC-to-DC converter 100 becomes complicated and is increased in size. Furthermore, the direct current voltage conversion unit 10 where the switching transistor Tr1 is operating abnormally continues its operation in parallel in the face of heat generation. Accordingly, it is necessary to provide means for stopping the operation of the failed direct current voltage conversion unit 10. In addition, it is necessary to provide switching means for switching operation to the redundant power supply circuit. Especially, in terms of the switching of operation to the redundant power supply circuit, the direct current power of a stable output voltage cannot be supplied immediately after the redundant power supply circuit is activated; accordingly, it is difficult to perform the switching control of switching operation to the redundant power supply circuit without stopping the supply of the direct current power to the load RL.
The present disclosure has been made considering such related problems. An objective thereof is to provide a DC-to-DC converter with a simple circuit configuration that continues the supply of direct current power to a load RL without providing a redundant power supply circuit even if a switching transistor Tr1 operates abnormally in the active state.
Moreover, another objective is to provide a DC-to-DC converter that has no risk of heat generation even if the switching transistor Tr1 of a direct current voltage conversion unit is caused to operate continuously in the active state.